This invention relates to an insulator or dielectric spacer for use in vacuum between a cathode and an anode, which may be two electrodes supplied either with an AC or DC voltage.
Although semiconductor devices are widely used, vacuum tubes are still indispensable. In such a vacuum tube, a voltage of a high tension, such as 100 kV, is supplied between a cathode and an anode with a dielectric spacer used to insulate the cathode and the anode from each other. The voltage develops an electric field of a strong field intensity, such as 100 kV/cm, along a spacer surface of the dielectric spacer. Such a high voltage and a strong electric field give rise to surface flashover or to objectionable surface leakage.
Various designs are in use to prevent the surface flashover from taking place. Examples are disclosed in Japanese Patent Prepublications (A) Nos. 106,745 of 1983, 255,642 of 1992, and 280,037 of 1992. The surface flashover is theoretically discussed in a paper contributed by J. M. Wetzer and another to the IEEE Transactions on Electrical Insulation, Volume 28, No. 4 (August 1993), pages 681 to 691, under the title of "The Effect of Insulator Charging on Breakdown and Conditioning" and a paper contributed by O. Yamamoto, one of two present joint inventors, and three others to the IEEE Transactions on Electrical Insulation, the same issue, pages 706 to 712, under the title of "Monte Carlo Simulation of Surface Charge on Angled Insulators in Vacuum".
In the manner which will later be described in greater detail, these conventional dielectric spacers are still objectionable. For example, a conventional dielectric spacer is bulky, is complicated in its shape, is expensive to manufacture, or does not have a well-developed design mechanism.